Zone 6b, in the northernmost part of the state, around the cities of Amarillo, Canyon and Hereford, averages 18 inches of snow per year. Winter temperatures can drop to temperatures as low as -10 to -5 °F. Plants from this region are most commonly grown in North Texas states. The coldest area indicated on the Texas USDA plantation map is 6b, located in the northern part of the status.
The rest of Texas enjoys very warm weather all year round, including the winter months. The remaining areas of Texas include zones 7a, 7b, 8a, 8b, 9a, 10a, and 10b. Winter lows can drop to -5 degrees F. or reach 20 degrees F.Below is our Texas section showing that Rockwall County is in Zone 8b.
Follow this link to the USDA site. Dallas-area gardeners have known for years that the region is warmer than its official USDA Zone 7 status. While the Texas zone map doesn't guarantee plant survival, it does provide a very solid framework for planning a garden. He said that, although much of the country is in warmer areas, the map “simply isn't a good instrument for demonstrating climate change because it's based only on the coldest days of the year. Downtown Dallas, that is, Dallas, approximately south of the LBJ Highway, was considered Zone 8a for more than a decade by local gardening experts.
The USDA spokeswoman, Kim Kaplan, who was part of the team responsible for the map, tried repeatedly to distance the new areas on the map from problems related to global warming.